Soil temperature for planting

Why soil temperature matters

Germination is a chemical process that speeds up as the soil warms, within limits. In cold soil, seeds of warm-season crops take much longer to sprout, or fail. While they wait, they are exposed to soil fungi that cause seed rot and damping-off. Even transplants suffer in cold soil. Roots grow slowly, the plant can’t take up nutrients well, and tomatoes and peppers can stall for weeks. A plant set out a week later into warm soil often catches up and overtakes one planted early into cold ground.

Air temperature is a poor guide because soil warms more slowly than air in spring, and stays warm longer in fall. Your last frost date tells you when frost is unlikely. It doesn’t tell you whether the soil is warm enough. Soil temperature is the second check.

How to measure it

• Use a soil thermometer with a metal probe. They are inexpensive at garden centres.
• Measure at planting depth. About 2 inches deep for small seeds, and about 4 inches for large seeds and transplants. Four inches is a common standard depth for soil temperature readings.
• Measure in the morning. Soil is coolest around sunrise and warms through the day. A morning reading is the conservative one.
• Take a few days of readings. One warm afternoon doesn’t mean the soil has warmed. Look for several mornings in a row at or above the minimum.
• Measure where you will plant. A raised bed in full sun can be several degrees warmer than a shaded in-ground bed a few yards away.

Minimum soil temperatures

From our crop table. These are approximate minimum soil temperatures for reasonable germination. Most crops germinate faster and more reliably when it is warmer than this. For transplants, treat the number as the minimum before planting out.

A few rough groups stand out:

• Around 40°F: peas, spinach, lettuce, radish, arugula, garlic. These can be sown in early spring.
• Around 45–50°F: the cabbage family, carrots, beets, onions, chard, potatoes.
• Around 60°F: tomatoes, beans, corn, basil.
• 65–70°F: peppers, eggplant, squash, cucumbers, melons, okra and many summer flowers. These are the crops most often planted too early.

Warming cold soil

• Raised beds warm faster in spring because they drain better and have more surface exposed to the sun.
• Cover the soil with clear or dark plastic for a week or two before planting. Clear plastic generally warms soil more but lets weeds grow underneath. Dark plastic also suppresses weeds. Many growers plant warm-season crops through holes cut in dark plastic mulch.
• Hold off on mulch. Straw and other organic mulches keep soil cool, which is useful in summer but slows spring warming. Apply them after the soil has warmed.
• Improve drainage. Wet soil warms slowly. Adding organic matter over time helps.
• Use cloches, tunnels or cold frames to warm both the air and the soil around early plantings.

Too warm is a thing too

Some cool-season seeds germinate poorly in hot soil. Lettuce and spinach are well known for this. Sowing them in midsummer for a fall crop can be tricky. Gardeners work around it by sowing in the shade of taller crops, keeping the seedbed moist and shaded with a board or shade cloth until seedlings appear, or starting seeds indoors and transplanting.

Soil temperature and frost dates together

Use both checks:

1. Is it past the right point relative to your last frost? Hardy crops can go in weeks before it. Tender crops go in after it. The crop timing guide and your planting calendar give the window.
2. Is the soil at least the minimum temperature at planting depth, on several mornings in a row?

If the answer to both is yes, plant. If the calendar says yes but the soil says no, wait a few days, or warm the soil first. Our tools work from frost dates because they are available for every ZIP code from long-term records. Soil temperature has to be measured in your own garden.

Related

• Cool- vs. warm-season crops
• Starting seeds indoors
• Hardening off seedlings
• How frost dates work
• Frost dates for your ZIP

Written for SowByZip. Last reviewed 2026-10-20. See about this site for how pages are made.